British Patent Specification No. 2082941 and our European Patent Specification No. 0162650 each relate to a separator, for separating components of a fluid mixture. In particular, these two prior patent specifications relate to devices which are particularly suitable for separating sewage and other solid matter from water in storm water overflows. It is a prime consideration, therefore, in these prior specifications, that the separators should have the highest possible efficiency of separation whereby the clean water which is removed from the top of the vessel has substantially all of the solid material separated therefrom. With these earlier patent specifications, it is always the case that some very fine particles of matter are carried over with the clean water at the top of the vessel.
U.S. Pat. No. 4,146,468 describes a liquid-solid cylindrical separator having a flat base in which solids are separated under the action of gravity and are swept towards a central outlet in the base under the influence of a forced vortex. Liquid is withdrawn from the top of the separator to establish a free vortex in which a relatively light fraction (including floatable material) is concentrated. In contrast with our earlier patent specifications, the device described in U.S. Pat. No. 4,146,468 includes neither a flow modifying member which defines an annular slot with the base nor an annular dip plate for stabilizing flow patterns in the separator.
In the devices the subject of the earlier patent specifications, there are several flow patterns in operation. Firstly, there is a general rotation of fluid in the vessel about the axis of the vessel and, secondly, a toroidal flow generally rising upwardly at the centre of the vessel and moving generally downwardly at the periphery of the vessel. Solid material entering the vessel generally sinks under the action of gravity and is swept toward an outlet at the centre of the base.
In order to achieve efficient separation, it has been found that a flow modifying member, an annular dip plate and a top baffle which restrains material flowing upwardly at the centre from flowing directly out of the vessel, are essential to the efficient running of the device.